In the Release Notes for the newest Tidy is this paragraph:
Andre Hinrichs wanted Tidy to do a better job on font
elements with relative size changes. This is in fact rather
tricky. Currently, Tidy uses percentage scaling values for
fonts rather than the enumeration defined by CSS [xx-small |
x-small | small | medium | large | x-large | xx-large]. The
first problem is to match these 7 values onto the 6 define by
the font element. The next problem is caused by the fact that
CSS doesn't provide matching relative font size values that
you could match to the ones defined for the font element. I
have done my best using percentage values, base on tests with
IE and Navigator. If anyone can come up with a better
approach, please let me know.
Todd Fahrner has tackled with this issue like no one else. See
http://www.deja.com/=dnc/getdoc.xp?AN=539168508
and in particular (mentioned at the bottom of the above):
http://style.verso.com/font_size_intervals/altintervals.html
/Jelks